Photosensitive dyes are coated on metal oxide films to render devices, such as solar cells, effective in the conversion of visible light to electric energy. In such a solar cell, a monolayer of dye is attached to the surface of a nanocrystalline metal dioxide film. Most efficient organic sensitizers contain a donor and an acceptor bridged by a π-conjugation linker (D-π-A). A great variety of organic sensitizers based on coumarin, merocyanine, indoline, polyene, hemicyanine, triphenylamine, fluorene tetrahydroquinoline, porphyrin, and carbazole moieties as a donor group give respectable conversion efficiencies in dye-sensitized solar cells. Most of the D-π-A dyes have carboxylic acid, cyanoacrylic acid or rhodanine-3-acetic acid moieties as electron acceptors, and also as anchoring groups for attachment on TiO2 surfaces. Carboxy groups can form ester linkages with TiO2 surfaces to provide strongly bound dyes and good electron communication. The photoabsorption properties of a D-π-A dye are associated with intramolecular charge transfer (ICT) excitation from the donor to the acceptor moiety of the dye, resulting in efficient electron transfer through the acceptor moiety (carboxy group) from the excited dye into the TiO2 conduction band. The charge transfer or separation between the electron donor and acceptor moieties in the excited dye may facilitate rapid electron injection from the dye molecule into the conduction band of the TiO2, so that it would be expected to separate the cationic charge effectively from the TiO2 surface and to restrict recombination between the injected electron and the oxidized dye efficiently.
As a noteworthy structural feature of D-π-A dyes, the highest occupied molecular orbitals (HOMOs) are in many cases delocalized over the n-conjugated systems in configurations centering on donor parts, while the lowest unoccupied molecular orbitals (LUMOs) are delocalized over acceptor and anchor parts. The photoinduced electron transfer from D-π-A dyes to TiO2 electrodes can thus efficiently occur by ICT with respect to HOMO-to-LUMO transition. Photoexcitation of the dye results in the injection of an electron into the conduction band of the metal oxide. The original state of the dye is subsequently restored by electron donation from a redox system, such as iodide/triiodide couple. Molecular design of new photosensitizers for nanocrystalline TiO2 film in solar cell that can absorb visible lights of all colors presents a challenging task. The absorption spectra of organic dyes could be red-shifted by expansion of the it conjugation in the dyes and introduction of electron-donating and—accepting substituents into the dye skeletons. The dyes should have suitable ground- and excited state redox properties so that the charge injection and regeneration of the dye occur efficiently.
As relevant art is mentioned in Adv. Funct. Mater. 2011, 21, 756-763.